Traditionally polymer foams are manufactured using mostly hydrophobic thermoplastic materials such as Polyurethane, PVC and Polyethylene, which require high processing temperatures. Prior to forming the foam a gas-filled polymer must be formed. There are a number of typical methods of forming a gas-filled polymer which include thermal decomposition of chemical blowing agents, mechanical whipping of gases into a polymer melt, use of low boiling point liquids, expansion of a gas dissolved in a polymer upon reduction of pressure in the system and the incorporation of microspheres into a polymer mass. Thermal decomposition of chemical blowing agents can be achieved by application of heat or as a result of the exothermic heat of reaction during polymerisation. The decomposition generates a gas such as N2 or CO2, which forms a gas-filled polymer. Incorporation of microspheres into a polymer mass relies on the incorporation of microspheres of a first gas-filled polymer into a polymer mass followed by heating of the mixture to cause expansion of the first gas-filled polymer to form a second gas-filled polymer.
After obtaining the gas-filled polymer by the any of the methods described above, the foam is then formed using one of three common manufacturing processes: compression moulding, reaction injection moulding or extrusion of the foam. The temperatures involved in these processes can be very high, e.g. in excess of 150° C., as the polymers used are in their molten state.
The most common processing method used in creating polymer foam is extrusion. This is a three-stage operation consisting of forming a polymer solution with gas dissolved in it to create a single-phase solution. The gas may be dissolved by injection of N2 or CO2, or by the use of blowing agent. Nucleation sites are then formed, as a result of a rapid pressure drop to create large numbers of uniform sites. Cell growth then takes place by means of diffusion of the gas to form bubbles. Processing conditions provide pressure and temperature changes necessary to control cell growth.
Our co-pending UK Patent Application Number 0218507.2 entitled A METHOD OF MAKING A MATERIAL describes a method of making polymer foams using very low processing temperatures in comparison to conventional methods. In the method a polymer foam can be created in two ways. In a first way voids are produced by generating gas in an aqueous coating solution of a hydrophilic polymer. In a second way an aqueous solution of hydrophilic polymer is coated onto a support, voids being created in the solution after the coating process during the setting and drying of the foam.
To create voids by generating gas in the coating solution, a coating solution is prepared comprising an aqueous solution of a polymer and blowing agent, the solution being treated so that micro-bubbles are formed therein. The coating solution, being a liquid composition containing micro-bubbles, is then coated onto a support. The micro bubbles may be formed in the coating solution by heating the solution to promote the decomposition of the blowing agent to form a gas, or by the addition of an acid to react with the blowing agent to again form a gas within the solution.
To generate voids post coating during the setting and drying process, a solution comprising a polymer and a blowing agent is coated onto a support. The coated support is then heated to a suitable temperature to cause activation of the blowing agent during the setting and drying of the coating.
The size of the bubbles formed in the solution and consequently in the foamed material can affect the physical properties of the foam. For example, the size of the bubbles affects the absorbency of the foamed material. This is important when the material is used as an inkjet-recording medium.
U.S. Patent Publication Number 2002/0057323 in the names of Yasuo Kurachi, Eiichi Ueda, Takayuki Sasaki and Akihisa Nakajima entitled INKJET-RECORDING MATERIAL, relates to a glossy voided inkjet receiver, which combines good drytime and water resistance with minimal cracking. It discloses a 2-layer product where at least 30% of the voids in the material are formed by using a foaming reaction in a layer comprised of organic materials such as synthetic resin monomers and epoxy resins.
The article entitled “Formation of Polymeric Foams From Aqueous Foams Stabilised Using A Polymerisable Surfactant” by Palani Raj W R, Sasthav M, Cheung H M, of Akron University, published in the Journal of Applied Polymer Science; 49, No 8, 20th August 1993, P. 1453-70 describes the creation of stable polymeric foams by purging an aqueous system (made up of a hydrophobic monomer, a polymerisable surfactant and a crosslinking agent) with nitrogen.